Page 19 - Curriculum Visions Dynamic Book

Selenium (Se)
Element 34. Selenium is a metalloid (between a metal and a non-metal) in group 6 (the oxygen group) in the Periodic Table.
It occurs in two forms: one has crystals and is grey, and one is glassy and black when solid and red when in powdered form.The crystalline and glassy forms have different densities and melting points.
It has the unusual property
of conducting electricity much better when light falls on it. It can also convert light into electricity.
Discovery
It was discovered in 1818 by Swedish chemist Jöns Jacob Berzelius.
Technology
It is used in photoelectric cells and solar cells. Selenium can convert AC electricity to DC and is therefore used in rectifiers. It also makes red enamels and strengthens rubber. It produces an alloy with stainless steels.
Geology
Selenium can occur as a native element associated with native sulphur. It is extracted as a by-product of copper refining. It does not readily form minerals.
Biology
Selenium is an essential trace element
and so is a vital part of all diets.
However, high levels of selenium can be poisonous. Plants growing in high selenium soils have poisoned grazing animals.
 Selenium is used in photoelectric cells.
Key facts...
Name: selenium
Symbol: Se
Atomic number: 34
Atomic weight: 78.96
Position in Periodic Table: group 6 (16)
(oxygen group; chalcogen); period 4
State at room temperature: solid
Colour: red, black, grey
Densities of solids: glassy 4.28 g/cc; grey 4.79 g/cc Melting points: glassy 50°C; grey 217°C
Boiling point: 685°C
Origin of name: from the Greek word selene,
meaning moon
Shell pattern of electrons: 2–8–18–6
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